Seismic data to analyse the Seti River landslide.

Netra Prakash Bhandary (May 22 2012)

This is false interpretation of the cause of SETI River Disasters. The landslide damming story is completely wrong because the debris has travelled a distance of 17 km from the source (i. e., the avalanche zone) in about 40 minutes. We conducted an aerial survey recently, but found no traces of any pounding of water or debris. So, the sole cause is debris-mixed avalanche, which seems to have muddified/slurrified immediately after failure, probably because of the frictional movement of the debris-mixed ice blocks. Another possibility is that the avalanche also forced-broke the debris-mixed ice mountains (which we can still see in the source zone of SETI River, which also turned into debris slurry within a few minutes. We interviewed the Russian pilot, who told us that he saw the huge avalanche cloud around 9:00 AM, 5th May. The photo taken by the picnicking college boys indicates the time of their photo to be 9:38 AM, which means the debris travelled up to Kharapani (Tatopani) in just about 40 minutes. How come we can think of landslide damming or an LDOF? We are simply disseminating misleading information without doing necessary groundwork. Now, some westerners are also talking of another rubbish by raising the issue of seismically induced landslide or avalanche. There are chances that an imperceptible seismic event took place and induced the avalanche, but this particular event may rather be a geological process, which may be typical to the Himalayas and snow-covered mountains. What may happen is because of the continuous snow cover as well as variation in temperature, the slope material undergoes deterioration in a long period of time, say about 50-100 years. As the snow cover may go as high as a few tens of meters, the pressure on the deteriorated slope material rises causing possible creep of the slope material together with the glacier movement. After a certain time, particularly after physical force imbalance, some part of the slope or the snow cover fails and causes a heavy slide of the weakened part of the slope or snow cover. This may happen when the iced mass of snow cover is close to melting point, i. e., less than 0 degree Celsius. We found that on 19 May, the melting line was somewhere around 4,000m of altitude, which indicates that the snow cover around this altitude is close to melting point. On top of that, when some foreign materials are mixed in the ice, during sliding or movement, the ice may melt faster due to frictional heat. This might have happened in this particular case. However, this speculation also requires some experimental verification, which we plan to study further in the days ahead.

Robert Simmon (May 28 2012)

Dr. Stark isn't claiming seismicity triggered the landslide, he used seismic data to detect it. The location of the landslide was confirmed by later Landsat data: http://blogs.agu.org/landslideblog/2012/05/23/understanding-the-seti-river-landslide-in-nepal/

Colin Stark (May 29 2012)

Indeed, Dr Bhandary has misunderstood our analysis of the seismic data. The landslide appears to have generated strong seismicity, a significant portion of which was long-period surface waves detected at several broadband seismometers at great distances from the source. Slope failure was emphatically not generated by a tectonic earthquake. Analysis of the teleseismic data revealed that failure took place at 9:09:56am +-15s. The direction of motion of the main mass was to the west for about 2000m or so, with runout extending rather further but with less than the full mass. Our evaluation of available satellite imagery indicates that failure of the ridge ~2km south of Annapurna IV was the source of the main landslide mass. Therefore Dr Bhandary is correct in several respects - about the timing and the source - but not about the seismology.